Environ. Sci. Technol. 2010, 44, 8102–8107
Pyrene and Phenanthrene Sorption to Model and Natural Geosorbents in Single- and Binary-Solute Systems ´ QUARIS,* JING ZHANG, JEAN-MARIE SE HANS-DIETER NARRES, HARRY VEREECKEN, AND ERWIN KLUMPP Agrosphere Institute, ICG 4, Forschungszentrum Ju ¨ lich GmbH, D-52425 Ju ¨ lich, Germany
Received April 6, 2010. Revised manuscript received August 25, 2010. Accepted September 10, 2010.
Sorption of pyrene and phenanthrene to model (illite and charcoal) and natural (Yangtze sediment) geosorbents were investigated by batch techniques using fluorescence spectroscopy. A higher adsorption of phenanthrene was observed with all sorbents, which is related to the better accessibility of smaller molecules to micropores in the molecular sieve sorbents. In addition, pyrene sorption in binary-solute systems with a constant initial concentration of phenanthrene (0.1 µmol L-1 or 2 µmol L-1) was studied. A 0.1 µmol L-1 concentration of phenanthrene causes no competitive effect onthepyrenesorption.A2µmolL-1 concentrationofphenanthrene significantly suppresses the sorption of pyrene, especially in the low concentration range; nonlinearity of the pyrene sorption isotherms thus decreases. The competitive effect of 2 µmol L-1 phenanthrene on the pyrene sorption is overestimated by the ideal adsorbed solution theory (IAST) using the fitted single sorption results of both solutes. An adjustment of the IAST application by taking into account the molecular sieve effect is proposed, which notably improves the IAST prediction for the competitive effect.
Introduction A large body of research has been performed on polycyclic aromatic hydrocarbon (PAH) sorption to soils and sediments. The sorption mode and extent depend largely on the content and type of soil or sediment organic matter (SOM). The sorption isotherms found in these studies deviate from linearity, especially when low concentrations of PAH are studied and/or condensed organic matter is present (1, 2). The Freundlich model can be conveniently applied to describe the nonlinear sorption isotherms. According to the modern conceptual model of organic carbon (OC) composed of amorphous organic carbon (AOC) and condensed carbon, mainly black carbon (BC) in surface soils or sediments, the sorption of PAH to SOM can be alternatively described by composite sorption models (1, 3), which contribute to the understanding of the sorption mechanisms. The PAH sorption into the AOC phase is a partition process without significant competitive effect (4). The partition coefficient is substantially related to the hydrophobicity of both sorbent and sorbate. Adsorption to the BC matrix is more specific. * Corresponding author phone: +49 2461 616751; fax: +49 2461 612518; e-mail:
[email protected]. 8102
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A steric effect (i.e., accessibility to the adsorption sites) must be taken into account in addition to the affinity effect (i.e., hydrophobicity). In fact, previous studies on the sorbate effect have demonstrated a lower adsorption capacity for larger and less planar molecules (5-8), suggesting an important role of the steric factors in the case of adsorption to BC. In this context, investigation of PAH sorption in binarysolute systems can provide complementary information in order to understand the sorption mechanisms and evaluate the adsorption sites. Introducing a cosolute into a system could modify the accessibility and affinity of the adsorption sites for the primary solute. In the previous studies on the cosolute effect of PAH or other organic chemicals, a competitive or cooperative effect of the cosolute on the sorption of the primary solute has been observed depending on the properties and concentrations of both solutes, as well as on the nature of the sorbents (9-11). The ideal adsorbed solution theory (IAST) has been considered for predicting the competitive adsorption of organic chemicals in multisolute systems (4, 12). As indicated elsewhere (4, 12, 13), one of the underlying assumptions of the IAST is that the sorbent presents the same adsorption sites to all solutes. This is not fulfilled for a molecular sieve sorbent where the different sites available for adsorption depend on the sorbate properties, e.g., sorbate molecular size (13). Xing et al. (12) found that the IAST failed to predict competitive adsorption on silica in the binary-solute system of atrazine and trichloroethylene, because silica does not present the same adsorption sites for both solutes. The main objective of this work was to investigate the cosolute effect of phenanthrene on the sorption of primary solute pyrene to model geosorbents and a natural geosorbent, sediment from the Yangtze River. Not only was the pristine sediment studied, but also the preheated sediment according to the CTO-375 method (14). This allows differentiating the contribution of AOC and BC to the PAH sorption in the pristine sediment (15). Charcoal and illite served as model sorbents for BC and clay minerals, respectively. Pyrene and phenanthrene sorption in single-solute systems was investigated and compared. In binary-solute systems, the effect of 0.1 µmol L-1 and 2 µmol L-1 phenanthrene as cosolute on the pyrene sorption was systematically studied. Freundlich and Polanyi-based models were applied to fit the sorption isotherms. The aim of this study was also to predict the sorption in a binary-solute system from the fitted singlesolute sorption results. An adjustment of the IAST fitting is thus proposed, taking into account the molecular sieve effect measured from the single-solute sorption results. The new conceptualization and solution approach for the IAST proposed recently by Benjamin (16) was applied.
Materials and Methods Materials. Pyrene (99%) and phenanthrene (99.5%) were purchased from Sigma-Aldrich. Selected physical and chemical properties are listed in Table S1 (Supporting Information). Stock solutions were prepared in ethanol. The surface sediment (NJ1) was collected in May 2007 from the bank of Yangtze River in Nanjing (32°7′2′′N, 118°44′1′′E). After collection, the materials were dried at 60 °C and then passed through a 2-mm sieve and finally homogenized by graining. One part of the NJ1 sample preheated at 375 °C under air according to the CTO-375 method (14) is labeled as NJ1375. Illite was purchased from Cs-Ker Illit Bt (Bekecs, Hungary). Prior to application in sorption experiments, illite was fractionated by sedimentation in water to remove the large particles (>2 µm). Charcoal was 10.1021/es1010847
2010 American Chemical Society
Published on Web 10/01/2010
obtained from burnt beech wood. The bulk materials were ground and fractionated. The fraction of
